[rpc] add initialblockdownload to getblockchaininfo

[dagurval]

Mimics the initialblockdownload added to getblockchaininfo in https://github.com/bitcoin/bitcoin/pull/11258.

I couldn't find a IBD estimator in the bcash codebase. This is probably not a good check for IBD status, but it's only supposed to be an estimate anyway.

[codecov-io]

Codecov Report

Merging #131 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #131   +/-   ##
=======================================
  Coverage   55.55%   55.55%           
=======================================
  Files         109      109           
  Lines       27184    27184           
  Branches     4498     4498           
=======================================
  Hits        15102    15102           
  Misses      12082    12082

Impacted Files	Coverage Δ
lib/node/rpc.js	22.54% <ø> (ø)	:arrow_up:

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[pinheadmz]

<= 0.99 might be ok, but also note if there hasn't been a block in about 40 minutes, bcoin will start to show progress as < 1:

https://github.com/bcoin-org/bcoin/blob/f585d8616df8335fddbc21d1b776a979ec6ff305/lib/blockchain/chain.js#L2116-L2126

[dagurval]

<= 0.99 might be ok, but also note if there hasn't been a block in about 40 minutes, bcoin will start to show progress as < 1:

https://github.com/bcoin-org/bcoin/blob/f585d8616df8335fddbc21d1b776a979ec6ff305/lib/blockchain/chain.js#L2116-L2126

I put the numbers in a sheet and as you can see this doesn't seem to be an issue.